The present invention relates to a light diffusing coating which is applied to the exterior surface of the glass bulbs of electric lamps and in particular tungsten halogen incandescent lamps.
Light diffusing coatings are used in incandescent lighting applications to reduce glare from the hot filament and produce a softer, more even illumination. Typically, the coating is applied to the interior surface of the lamp envelope. However, in some lamp applications, it is preferable to have the coating on the exterior of the lamp, e.g., tungsten halogen lamps. Such coatings must be uniform and diffuse the light without effecting a substantial loss in lumen output. In particular, it is desirable that the loss in luminous flux be less than about 10%. The coating must have sufficient mechanical strength to prevent damage during handling and lamp operation and should not adversely impact lamp life. It is also desirable that the coating process be compatible with conventional manufacturing techniques to keep costs low.
One well known light diffusing coating is composed of fumed silica (silica xe2x80x9csmokexe2x80x9d). Fumed silica typically is formed by flame hydrolysis of silicon tetrachloride and is composed of extremely fine particles of silica having a diameter less than about 100 nm. The silica xe2x80x9csmokexe2x80x9d is adhered to the inner bulb surface by an electrostatic attraction between the glass surface and the silica particles. However, silica xe2x80x9csmokexe2x80x9d coatings are not suitable for use on exterior bulb surfaces since they lack sufficient mechanical strength and are degraded by moisture.
Another well known method of providing light diffusion is acid etching or xe2x80x9cfrostingxe2x80x9d of the surface of the glass bulb with hydrofluoric acid (HF). While HF frosting provides an effective light diffusing surface, there are drawbacks associated with this method which limit its use. Particularly, the acid etching adversely affects bulb strength and the use of hydrofluoric acid raises important environmental, health and safety concerns.
An alternative to HF frosting is sandblasting which can produce an appearance similar to frosted bulbs without the problems which accompany the use of hydrofluoric acid. Sandblasting causes the formation of fractured regions (defects) on the glass surface which act as light scattering centers. However, sandblasting also diminishes bulb strength and introduces contaminates which promote devitrification of the glass during operation. This is a particular problem at the high bulb temperatures observed in operating tungsten halogen lamps. Furthermore, methods which result in diminished bulb strength are not desirable for tungsten halogen lamps which have operating pressures of several atmospheres.
Hence, it would be an advantage to have a durable, light diffusing coating which could be applied to an exterior bulb surface without diminishing the strength of the bulb.
It is an object of the invention to obviate the disadvantages of the prior art.
It is another object of the invention to provide a durable, light diffusing coating for use on the exterior surfaces of the glass envelopes of electric lamps.
It is a further object of the invention to provide a coating solution for applying a durable, light diffusing coating to an electric lamp and a method for applying the coating solution which is compatible with conventional lamp manufacturing techniques.
In accordance with one aspect of the invention, there is provided a light diffusing coating for an electric lamp having a glass bulb comprising: fine silica particles having an average diameter of from about 0.5 to about 4 micrometers and ultrafine silica particles having an average diameter of from about 10 to about 100 nanometers; the fine and ultrafine silica particles being adhered to the glass bulb by a sol gel derived silica.
In accordance with another aspect of the invention, there is provided a sol gel coating solution for forming a light diffusing coating on an electric lamp comprising: fine silica particles having an average diameter of from about 0.5 to about 4 micrometers and ultrafine silica particles having an average diameter of from about 10 to about 100 nanometers, said silica particles being suspended in an alcoholic solution of tetraethoxysiliane and diethoxydimethylsilane.
In accordance with yet another aspect of the invention, there is provided an electric lamp having a filament, lead wires, and a glass envelope; the glass envelope containing the filament and having a light diffusing coating on an exterior surface; the filament being connected to the lead wires and connectable to an external source of electrical energy; the light diffusing coating comprising: fine silica particles having an average diameter of from about 0.5 to about 4 micrometers and ultrafine silica particles having an average diameter of from about 10 to about 100 nanometers; the fine and ultrafine silica particles being adhered to the glass envelope by a sol gel derived silica.